Despite the repeated appeals by health and environmental authorities, the volume of municipal solid waste and of the wastes which may be assimilated thereto keeps growing everywhere, both because the well-being currently found in parts of the planet implies constantly-growing consumptions, and because the same health authorities require bulkier packagings and often the use of disposable items, especially in case a high level of hygiene is required. Waste disposal is thus a general and invasive problem, particularly in the most industrialised countries where the above-mentioned problems are often matched by lack of space.
So far, also due to the population's problems of acceptance towards the other disposal methods, wastes have been mostly dumped in controlled landfills. Such system, however, is increasingly inadequate, since it requires to constantly find ample spaces, possibly far from residential areas, and causes various forms of environmental pollution; in particular, the liquids originating from waste decomposition and containing various even highly toxic substances, often seep into the water-bearing layer.
The incineration of municipal solid waste, a practice once a source of various toxic gases, is now carried out with greater care, so as to produce virtually only carbon-dioxide and water fumes, which would develop anyway over time even in a landfill. The improvement in the management of these processes, as well as the use thereof for heat and/or electric energy generation, is a reality in the most advanced countries and it is auspicable for it to become increasingly widespread.
At the end of the incineration, between 20% and 30% of the supplied waste remains in the form of bottom ash and/or debris, essentially consisting of the non-combustible fractions and made up mainly by vitreous and/or ceramic materials, coming from the chemical-physical transformations of the combustion process, especially for high temperatures (about 900° C.) which are reached in a correctly-managed incinerator, so as to avoid the building of dioxin and the like. A non-negligible percentage of the bottom ash and/or debris, about 5-20% by weight of the total, consists of material of the ferromagnetic type (metals and non-metals), but there are also metals of the paramagnetic and diamagnetic type.
In Italy and in Europe, bottom ash and/or debris from municipal solid waste incineration and waste assimilated thereto and from RDF (refuse-derived fuel) are classified as non-hazardous waste according to code CER 190112. Based on domestic provisions or specific authorisations, they may possibly be mixed with other same-origin waste.
Such bottom ash and/or debris are normally disposed of in a landfill. Proposals have also been put forward for reusing bottom ash and/or debris—the quantity whereof is in any case decidedly substantial—with special reference to the use thereof as building material.
Some of these proposals concern the use of bottom ash or debris as raw material for clinker production and hence of cement (often referred to as “ECOCEMENT”). This process is limited by the fact that bottom ash, coming from the thermal incineration process (at about 900° C.), must undergo a new thermal process, that of clinkerisation, at 1400-1500° C., to be reused and that does not appear particularly advantageous from an energy-impact point of view, as well as from the point of view of the economic and environmental balance.
Other studies show how bottom ash and/or debris, mainly consisting—as seen—of vitreous and ceramic components, have a pozzolanic potential. However, said bottom ash and/or debris, in addition to being coarse from a granulometric point of view and being rich in metals and various impurities, also contain non-negligible amounts of amphoteric metals, mainly aluminium, which, in a basic environment end hence, even more so, in the water-cement mixtures, originate complex ions and, hence, gaseous hydrogen which expands and, in the last analysis, may completely spoil the finished cementitious product, creating bubbles inside and in some cases possibly causing even hazardous situations.
Also the use of this bottom ash for the building of road beds, or for similar applications is not convincing, both for reasons of an environmental nature, and for the same phenomenon of aluminium corrosion with consequent hydrogen release and resulting expansion phenomena.
PCT WO 02/081 398 suggests wet shredding and aqueous suspension of bottom ash and/or debris to use them as additives for concrete and/or for cement conglomerates. However, since amphoteric metals (mainly aluminium, as said) are extremely ductile, much more so, in particular, than materials of a ceramic and/or vitreous nature which represent most of the ash, at the end of the described shredding in such document such metals remained in pieces of considerable size, so that they were unable to oxidise fully and the previously detected problems, connected to hydrogen release, remained unchanged.
EP 1 382 584 highlights that, during wet shredding of bottom ash and/or debris, corrosion by oxidation of the aluminium therein contained takes place; in order to overcome this, such document provides the preliminary shredding of the bottom ash in an aqueous suspension and the subsequent use thereof for concrete manufacture. However, afterwards, it was evident that simple wet shredding would not be sufficient to solve the problem of hydrogen formation. Moreover, even the subsequent addition of cement in order to raise the pH was not ideal, causing further problems.
With the shredding techniques used which have led both to PCT WO 02/081 398 and to EP 1 382 584, uncoupled with adequate separation and/or filtration phases, the particles of amphoteric metals were probably, in fact, so large (>300 micron) not to be detected during the measuring carried out by the laser granulometer capable of detecting only particles below 300 micron, or, since they represent a very small percentage, normally in the order of 1% by weight, they were quantitatively neglected.
Finally, also the attempts at separating beforehand the paramagnetic metals by special technologies, had produced appreciable results, but had not proven fully effective. In such respect tests had been carried out with equipment exploiting the principles of the so-called “Foucault currents” or detecting the electromagnetic alterations determined by the flow of the para-magnetic metals on conveyor belts, or by similar technologies.
Finally, also the recovery of the mainly metallic, ferro-magnetic fractions—which still did not have deleterious effects as in the case of the amphoteric metals—was not an ideal solution, since it allowed the recovery thereof only in limited percentages and did not allow, among other things, to obtain “clean” ferromagnetic metals, i.e. with a very low content of other fractions and/or impurities.